search for long-lived particles at dØ todd adams florida state university july 19, 2005 susy05 ippp...
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Search for Long-Lived Particles at DØ
Todd Adams
Florida State University
July 19, 2005SUSY05
IPPP Durham
July 19, 2005 SUSY05 – T. Adams 2
Tevatron
• pp collisions at 1960 GeV
• detectors: – DØ and CDF
recorded luminosity ~8 times Run I
_
July 19, 2005 SUSY05 – T. Adams 3
Introduction• Search for long-lived particles
– “stable” = doesn’t decay within the collider detector
• can still decay
– massive (>50 GeV)– charged particles = unique signature
• SUSY models– GMSB: stau– chargino– optimize for pair production
July 19, 2005 SUSY05 – T. Adams 4
GMSB Model• stable stau: NLSP (LSP is gravitino/goldstino)• large value of Cgrav
• Snowmass 2001 Model Line D
m scale of SUSY breaking 19 to 100 TeV
Mm messenger mass scale 2m
N5 number of messenger fields 3
tan ratio of Higgs VEVs 15
sign sign of higgsino mass term +1
Cgrav factor for effective mass of gravitino >>1
July 19, 2005 SUSY05 – T. Adams 5
Chargino Model
• stable chargino– higgsino-like
or– gaugino-like
higgsino-like gaugino-like (GeV) 60-300 10,000
M1 (GeV) 100,000 3M2
M2 (GeV) 100,000 60-300
M3 (GeV) 500 500
tan 15 15
squark mass (GeV)
800 800
• small (<150 MeV) mass difference between lightest chargino and lightest neutralino
July 19, 2005 SUSY05 – T. Adams 6
Charged Massive Stable Particles(CMSPs)
• Massive: >50 GeV • Slow-moving: < 1• Charged: large energy deposition
– Bethe-Block formula
• Long-lived: decays outside of muon detector• Will look like slow moving muon
22
2
2
420
1
2ln
4
I
mv
A
Z
mv
ezN
dx
dE
July 19, 2005 SUSY05 – T. Adams 7
DØ Detector
July 19, 2005 SUSY05 – T. Adams 8
Muon System• Detects minimum-ionizing particles• Timing: speed of light particle will register at
t=0 in all three layers– resolution is 2-3
ns
• Three layers• Toroid between
1st and 2nd layers• CMSP’s will be
out-of-time
July 19, 2005 SUSY05 – T. Adams 9
Trigger• Trigger requires two tracks in muon system
• Timing issues:– trigger gate is asymmetric
• allows later particles
– trigger gate ranges from 20-85 ns for different layers
– causes inefficiency for slower moving particles– efficiency drops as mass increases– sufficient for our mass range (60-300 GeV)
July 19, 2005 SUSY05 – T. Adams 10
Preliminary Dimuon Selection
• muon pT > 15 GeV
• matched to track in central tracker
• scintillator hits in 2 of 3 layers of muon system
• at least one isolated muon > 1.0 radians
• cosmic ray muon rejection
Luminosity = 390 pb-1
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Muon Speed
• Calculate speed of muon at each layer– v = d/t (v in units of c) v from timing error
• Calculate average velocity for each muon
• Calculate speed 2
• Require 2 < 4
layer
layeravg vv2
22 )(
July 19, 2005 SUSY05 – T. Adams 12
Speed Significance
Data muons
CMSP
July 19, 2005 SUSY05 – T. Adams 13
Backgrounds• Z/Drell-Yan is major background
– no natural source of slow-moving high-pT particles
• Use data to estimate background– use Z-peak events for timing distribution– use negative time
events for invariant mass distribution
July 19, 2005 SUSY05 – T. Adams 14
Transverse Momentum
Signal has larger pT
July 19, 2005 SUSY05 – T. Adams 15
Dimuon pT vs Significance Product
• Significance product: Sig(1) x Sig(2)
60 GeV stau’sReal muon data
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Analysis Results
• Background estimated using data
Stau Mass (GeV) Background Estimate Data Events
60 13.6 ± 0.7 ± 0.5 13
100 0.66 ± 0.06 ± 0.02 0
150 0.69 ± 0.05 ± 0.02 0
200 0.60 ± 0.04 ± 0.02 0
250 0.47 ± 0.03 ± 0.02 0
300 0.61 ± 0.05 ± 0.02 0
July 19, 2005 SUSY05 – T. Adams 17
stau Cross-section Limit
Best limit from the Tevatron
July 19, 2005 SUSY05 – T. Adams 18
CharginosCharginos look like stau’s
Use same analysis
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Chargino Limits
Higgsino-like Gaugino-like
M > 140 GeV M > 174 GeV
DØ RunII Preliminary
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Effect of Lifetime
• “Short” lifetime decay within detector
• loss of acceptance
July 19, 2005 SUSY05 – T. Adams 21
Conclusions• DØ has performed a search for
charged, massive stable particles– look for slow moving particles
• Set limits on stau and chargino production– best limits on chargino mass